Electric heating furnace



ct 1934- e. L. SIMPSON ELECTRIC HEATING FURNACE Filed Dec. 12, 1931 3Sheets-Sheet l Ivwerfir: 6 .11. Sa'wsaw,

@ct 2 11934. G. 1... SIMPSON ELECTRIC HEATING FURNACE Filed Dec. 12,1951 3 Sheets-Sheet 2 @ct 2 1134. G. L. SIMPSQN Y LWSAHQ) ELECTRICHEATING FURNAQE Filed Dec. 12, 1951 s Sheets-heet 3 i357. Z L

i mveaafar: .2. 56% $0M,

Patented? Got. 2, i934 1,t9'5,41 ELECTREQ HEATHNG FURNAUE George L.Simpson, Coraopolis, Pea, assignor to Pittsburgh Ea.

' is: .7 ch Corporation, Pittsburgh,

Application iillecernher 12, 1931, Serial No. %,639

it Clams (Cl. litw) My invention relates to improvements in clas trioheating furnaces and more particularly to improved means for heatingsuch furnaces.

Heat is transferred by radiation, conduction 5 and convection.Heretofore in the operation of this type of furnace it has been thepractice to depend almost entirely upon radiation for the transfer ofheat energy from the heating elements in the furnace to the charge.Convection id and conduction therefore play only a negligible part inthe electric heating furnace designs heretofore employed.

In the usual type of electric heating furnace, the heater elements donot completely cover the it? furnace walls. The average temperature oithe combination of walls and heater elements is consequently much lowerthan the maximum temperature or" the heater elements. This is due to thefact that where the heater elements do not completely cover the walls,the walls at a lower temperature are shining through the heater elementson to the work and they lower the average temperature of the radiatingsurfaces which deliver heat energy to the charge. As a result it isnecessary to heat the heating elements to a higher temperature thanwould be necessary if the heating element covered the entire wallsuriace of the furnace.

The nickel chrome alloys that are used for to most of the commercialheating elements of the present time have a melting point which limitsthe safe operating temperature of these elements. In the usual steelheat treating practice, temperatures in excess of 1800 C. were notrequired. The development in recent years, however, of the special alloysteels with corrosion resistant and heat proof properties andhighsilicon steels, has demanded higher and higher operating temperatures,and furnace builders have experienced ill considerable dimculties inproducing resistor elements that will heat a charge to 1900 or higherwithin a reasonable time without appreaching the softening point of theresistor, and causingthe failure of the heating element.

An important object of my invention, there= .fore is to provide aiurnacewhich can be open ated to produce the heat desired for present dayheating practice without endangering the re sistor element and withoutproducing uneven heating of the furnace charge.v

Another object of my invention is to produce even and eflicient heatingof a charge in an elec= trio heating furnace.

A iurther object of my invention is to provide a novel resistanceelement for use in electric heating furnaces.

In the accompanying drawings, which form a part of this specificationand wherein like char= acters of reference denote like or correspondingparts throughout the same, Figure l is a vertical sectional view of afur= nace embodying one form of my invention,

Figure 2 is a horizontal sectional view thereoi,

Figure 3 is a detail elevation of another form of resistor element,

Figure 1 is a similar view oi a further iorm of resistor element,

Figure '5 is an elevation of still another form of resistor,

Figure 6 is an edge elevation thereof,

Figure 7 is a vertical sectional view of another furnace embodying myinvention,

Figure 8 is an elevation of the resistor element used therein,

Figure 9 is an elevation thereof, parts being shown in section,

Figure 10 is a detail of a resistor element similar'to that shown inFigures '2, 8 and 9 but having strengthening ribs thereon.

Figure 11 is a transverse sectional view thereof,

Figure 12 is a vertical sectional view of still another form of furnace,I

Figure 13 is a horizontal sectional view thereof,

Figure 1% is a fragmentary sectional view showing a furnace having afurther modified resistor element therein, and,

Figure 15 is a fragmentary elevation oithe resistor element usedtherein.

Inthe drawings, wherein for the purpose of 11- lustration is shown apreferred embodiment of my invention, the numeral 16 designatesa heatingfurnace hood arranged about a base 1? and having a removable roof 1%.The base 17 preferably extends into the hood as shown, and in place ofthe usual heater elements arranged on the walls of the furnace hood, itprovide aresistance element comprising a flat plate 19 having a height Ihave shown the plate as being supported from the removable rooi 18 bymeans of the terminal posts 20 but it will be understood that any othersuitable supporting means may be employed.

It mil be seen that the charge arranged on 1w the base 1'7 will receiveradiation from the inner side of the plate 19 which substantiallycorresponds in size to the furnace walls. In addition to being heated byradiation, the charge will be subjected to heat by convection, due tothe s'pacan unbroken plate as in Figure 1, is slit at 22.

alternately from the top and bottom of the plate to form a series'ofU-shaped portions connected at their upper ends and with their armsarranged I very close together. This form of resistor is better suitedto high voltages and low current, in

line with present day practice. By slitting the,

plate as shown, the theoretical radiating surface is slightly reducedbut a sufiiciently low resistance is maintained in the heating elementto permit of higher voltages than required by the unbrokenresistorplate.

Due to convection and re-radiation from the roof of the furnace, andother factors, the top portion of a vertically arranged heater elementtends to run hotter than the lower portion. This situation isundesirable on account of the unequal temperatures'delivered to thecharge. This difliculty mayfberemedied by constructing the resistanceelement as shown'in Figures 4, 5, 6, 8

and 9 In the form of the invention shownin Figure 4, the resistor plate23 is cut-away to produce a resistor which has a larger surface at itsupper end than at its lower end. This result is accomplished byproviding lower slits 24. which increase in size as they approach thebottom of the plate and upper slits 25 which decrease in size as theyapproach the top of the plate. As the lower portion of the resistorplate has a smaller surface than the upper portion, it will consequent-1y have a; higher resistance, and will liberate more heat energy thanthe upper portion of the plate, thereby counteracting the tendency ofthe upper portion ,of the plate to be heated to a higher temperaturethan the rest of the plate.

In the form of resistor shown in Figures 5 and 6 the same result isobtained by forming the resistor from a plate 26 which has been rolledor cast thicker at its top than at its bottom and gradually decreasingin thickness towards its lower end. The'lower end having a smallercrosssection offers more resistance to the current and produces greaterheat than the thicker upper portion of the resistor, therebycounterbalancing the other factors tending to overheat the upper portionof the resistor plate.

In Figures 8 and 9 I have shown a resistor element 2'7 slit-similarly tothe plate of Figure 3 but having its upper unslit portion 28 bentinwardly at an'angle to form a radiating fin for providing greatercooling at the upper portion of the resistor as well as reducing theelectrical resistance in the top portion of the plate with a resultingreduction in the rate of heat liberation at this point.

In Figure '7 I have shown a furnace 29 which is preferably circular,although other shapes may be employed, and having a removable roof 30. Aresistance element such as that shown at 27 in Figures 8 and 9, isarranged in the furnace in I spaced relation to the walls thereof, and.extends from a point adjacent the bottom shelf of th furnace to apointadjacent the top thereof. In place of relying entirely upon'naturalconvection,

I provide a fan 31 supported by the roof through which the fan shaftextends, which produces a forced convection by causing a circulation asshown by the arrows. The fan motor 32 may be arranged in any suitableposition, such as being mounted on the top ofthe roof 30 and a fan guard33 may be provided to protect the fan.

It will be seen that the space between the resistor plate 27 and thewalls of the furnace will act as a chimney. The furnace atmosphere isforced down into this space, drawn up through the center of the resistorand again forced down through the space between the furnace walls andthe resistor plate. With thisarrangement for producing forcedconvection, the heat transfer from the heater element to the furnaceatmosphere and to the charge is greatly increased and consequently theresistor need not be heated to as high a temperature as would otherwisebe required. I have found in actual practice, for example, that in afurnace where no forced convection was used, a temperature difference ofapproximately 100 C. existed between the resistor element and thecharge. When, however, forced convection was utilized, as seen in Figure'7, the temperature difference between the resistor'and the charge wasreduced to 10 C.

The importance of this efficiency in heating is apparent when the demandfor high temperatures forces the operators to dangerously approach thepoint of softening and failure 'of the resistor, as in the annealing ofsigh silicon steel. With my flat resistor plate and forced convectiongreat heat liberation is effected without approaching the point offailure of the resistor. When heating a charge to a temperature of 1850"C. or 1900" C. in the usual type of furnace, a gradient of 100 C. to 150C. must exist between the temperature of the charge and of the resistor,if the heating is to be accomplished within a reasonable amount of time.This is due to the fact that radiation largely is utilized to transferthe heat energy in the usual furnace. By the use of forced convection,especially where a fiat resistor is used in place of the usual ribbon,the charge can be heated to these high temperatures in a short timewithout endangering the resistor element.

It will be obvious that in place of the resistor plate shown in Figure'7, any of the resistors shown in the other views may be used in thistype of furnace.

In Figures 10 and 11 the resistor plate, is'constructed similarly tothat shown in Figures 7, 8 and 9 with the addition of strengthening ribs35 arranged vertically of the arms of the resistor and pressed therein.-These ribs increase the strength and useful life of the resistorelement. In the furnace shown in Figures 12 and 13, the furnacestructure comprises a. furnace hood 29 and removable roof 30 with a fan31, and resistor 2'7 all identical with or similar to the structure setout in Figure 7. The base 36 however is provided with horizontal bores3'7 arranged above the shelf on the lower'end of the hood 29 andcommunicating witha vertical bore 38 arranged centrally of the base.This type of furnace is for use with a charge 39 of coils or othermaterial having a central opening therethrough. The fan 31 forces thefurnace atmosphere down through the space between the resistor 2'1 andthe hood 29, into the bores 3'7, up

In place of the flat resistors shown in Figures 1 to 13 I may employ aresistor such as shown in Figures 14 and 15. A resistor ribbon 40 iswound in close spirals about a relatively thin barrier such as a ceramicor other refractory plate 41 which extends arou'nd the furnace in spacedrelation to the walls of the, furnace hood. This form of resistor may beused with the type of furnace in which a fan islused, andit may alsobeused without a fan.

din the types of furnaces in which naturaland forced convection isnot'utilized, practically the only heat utilized is that radiatedfromthose por= tions of the heater element facing the charge. I Byforcing the furnace atmosphere to circulate between the walls of thefurnace and the heater element, however, I bring the heat from theportion of the heater element facing the walls of the furnace to thecharge. This results in the efiective utilization of heat usually mumedand to a large extent lost to the charge, and thereby nearly doubles theefiective heating capacity of the heater element.

An important feature of my invention is the use of the fanshown at 31 inFigures 7 and 12. it has been proposed to use a paddle type fan with theblades arralnged in a plane with the axis of the fan shaft in order toobtain peripheral disv charge of air from the fan. This arrangementhowever is disadvantageous due to the shock loss and eddy currentwastage occurring at the inlet of paddle or centrifugal ,type fans. Thiswastage is notpresent in the inclined blade or propeller type fandisclosed in Figures 7 and 12, but as heretofore used this type oftent-had the disadvantage of providing axial discharge of the airsubstantially parallel to the fan shaft. I

I propose to combine the advantages of a pro pellet type fan and acentrifugal fan by the provision of the guard or casing 33 shown inFigures 7 and 12 which gives axial inlet and peripheral discharge asindicated by the arrows in these figures. Withthe casing 33 therefore Iam able 'to obtain all of the advantages of peripheral the body of a boxtype or other types of furnaces,

as well as that type shown in the drawings, the

term hood being used in the claims as synonymous with body.

While I have shown and described the pre-.

' ferred embodiment of my invention, it is tube understood that variouschanges in the size, shape and arrangement of "parts may be resorted" towithout departing from the spirit of myinvention or the scope of thesubjoined claims.

Having thus described my invention, what I claim and desire to protect.by LettersPatent is; 1. An electric furnace comprising a base, a

furnace hood therefor, a removable roof for said hood, and a heatingelement supported by said roof and comprising a resistor plate extendingaround the walls of the furnace hood in. spaced relation therewith, andextending. to points adjacent to but spaced from the bottom of the hoodand the furnace roof.

2. An electric heating furnace comprising a furnace hood, a resistorelement arranged in spaced relation to the walls or" the hood andcomprising a plate having openings therein extending from its top andbottom, the openings extending from the bottom of the plate being largerthan the openings extending from the top of the plate whereby the platehas a greater surface adjacent its upper end than at its lower end.

3. An electric furnace comprising abase, a heating hood therefor, and aresistance element extending around the interior of the hood in spacedrelation to the walls thereof, and comprising a plate having slitsextending thereinto from the top and bottom thereof, said slits beingwidest at their lower ends and becoming narrower as they approach thetop of the plate, whereby the resistance plate has a greater surfaceatits upper end than at its lower end.

drA- resistance element for electric heating furnaces comprising a platecutaway to form a serleslof connected U-shaped portions, said platebeing progressively thicker from its lower to its upper end.

5.5m electric heating furnace comprising a furnace hood, a resistanceelement arranged therein and spaced from the walls thereof, saidresistance element comprising a plate cutaway to form a series ofconnected U-shaped portions, said Mil plate being progressively thickerfrom its lower...

end to its upper end.

6. An electric heating furnace comprising a furnace hood, a resistanceelement arranged therein and spaced from the walls thereof, saidresistance element comprising a plate having slits therein forming aseries of connected U-shaped portions, the top portions of saidLl-shaped portions being bent over at an angle to form cooling at thetop of the plate.

7. An electric heating furnace having a resistance element arrangedtherein and spaced from the walls thereof, said element comprising aplate slit to form a series of connected U-shaped portions, saidU-shaped portions being bent inwardly at their upper ends, and a fanarranged above the resistance plate and adapted to circulate the furnaceatmosphere around the resistance plate. 8. An electric furnacecomprising abase, a furnace hood arranged thereover, and beingsubstantially circular in cross section, a resistance element arrangedconcentrically within said hood and spaced from the walls thereof, saidresistance element comprising a plate extending from a point adjacentthe bottom of the hood to a point adjacent its top, means to pass anelectric current through, the resistance plate, and a fan arranged tocirculate the furnace atmosphere from the exterior of said plate to itsinterior.

' 9. An electric furnace comprising a furnace hood, a base extending upinto said hood and adapted to support a charge,said base having openingsextending therethrough, a resistance plate extending around said furnacehood interiorly thereof and spaced from the walls of the hood, and meansto circulate the furnace atmosphere around said resistance plate andthrough the openings in the base.

10. An electric furnace comprising afurnace hood, 9. base extending intosald'hood and adaptlate the atmosphere from the space between thefurnace g walls and resistor plate through the central opening in thecharge. r

V by the resistance plate.

11. An electriccfurnace having a resistance plate arranged therein andspaced from the walls thereof, and means to circulate the furnaceatmosphere in oppositely movingstreams separated 12. An electric furnaceadapted to receive a charge having a substantially centrally arrangedopening'therethrough, agresistance plate extending around said furnacewalls interiorly of the furnace and in spaced relation to thevfurn'a'cewalls, and a fan arranged to circulate the atmosphere from the spacebetween the furnace walls and resistor plate, through the centralopening in the charge.

13. An electric furnace having a resistance eleravaaio ment arrangedtherein and spaced from the walls thereof, and means to circulate thefurnace atmosphere in streams separated by the resistance casingarranged ahout said fan, said casing being provided with aninlet openingadjacent the axis of the fan and an outlet opening adjacent theperiphery of said fan. A

GEORGE L. smeson.

